When a round tube is continuously to be shaped in a plurality of shaping steps to a rectangular tube, it is known (FR-B 1,267,303) to provide in a rolling line a plurality of shaping stands, which are arranged in series and consist each of two pairs of diametrically oppositely arranged shaping rollers, the profiles of which constitute a substantially closed rolling contour. Because the radius of curvature of the profiles of the shaping rollers and, as a result, the radius of curvature of the associated portions of the rolling contour, is larger than the radius of curvature of the corresponding portion of the periphery of the tube as it enters a given stand, the tube is flattened in steps in the consecutive stands in the peripheral portions corresponding to the flat walls which are to be formed until the final rectangular shape has been achieved. Those known shaping stands have the disadvantage that the shaping rollers closely adjoin each other in the peripheral direction to define a closed shaping contour so that a change of the size of the rolling contour and an adaptation to different tube diameters cannot be effected unless the shaping rollers are replaced by shaping rollers having a correspondingly changed profile. The resulting alteration of the shaping stands involves a relatively large amount of work and also requires the entire shaping plant to be shut down.
In an attempt to avoid said disadvantages, a shaping apparatus has been disclosed (U.S. Pat. No. 3,347,078), in which the shaping rollers are arranged in pairs of diametrically opposite rollers and have eccentric profiles so that the diameter of each shaping roller gradually decreases from one end of the roller to the other and each shaping roller is adapted to be so displaced that its end which is larger in diameter approaches the adjacent shaping roller, which has an axis that is perpendicularly to the axis of the shaping roller thus displaced. In that case that end of the first-mentioned roller which is larger in diameter arrives adjacent to that end of the second roller which is smaller in diameter and, as a result, the profiles of the several shaping rollers, which profiles determine the rolling contour, overlap each other and the rolling contour is correspondingly reduced in size. This requires that each shaping roller is mounted in the stand to be adjustable in a radial direction relative to the tube to be shaped and in the direction of the axis of said roller. But the eccentric profile of each shaping roller necessitates that for an adjustment of the shaping rollers in adaptation to tubes having a different diameter the rolling contour must be rotated about the axis of the tube. Unless a twisting of the tube being shaped is permissible, it will be necessary to pivotally adjust the shaping rollers of each pair about the axis of the tube in opposite senses. This can be permitted if the shaping rollers are mounted in a rotatably mounted frame. Besides, each shaping roller can be pivotally adjusted individually so that the profile which determines the rolling contour can properly be aligned, particularly if that profile is composed of a plurality of sections corresponding each to an arc of a circle and having different radii. A disadvantage of that known apparatus resides mainly in its relatively large structural expenditure and in the fact that the tube is necessarily twisted as it is shaped unless special measures are taken.